There is no denying the left-leaning political bias on American college campuses. As data from UCLA's Higher Education Institute show, the professoriate has moved considerably leftward since the late 1980s, especially in the arts and humanities. In New England, where my own university is located, liberal professors outnumber their conservative colleagues by a ratio of 28:1.

How does this bias affect the education we offer? I'd like to think that we left-leaning professors are able to teach the works of conservative thinkers with the same seriousness and attention that we devote to works on our own side of the political spectrum—but do we?

It is hard to be optimistic about this challenge in the wake of recent episodes of campus intolerance for views on the right. Would-be social-justice warriors at Middlebury College transformed the mild-mannered political scientist Charles Murray into a free-speech hero, and campus appearances by the Manhattan Institute's Heather Mac Donald and the right-wing provocateur Ann Coulter have been handled badly, turning both women into media martyrs.

Most colleges, of course, host controversial speakers without incident and without much media coverage. In March, for instance, Franklin & Marshall College gave a platform to the Danish editor who published cartoons mocking the Prophet Muhammad. There were protests and arguments but no attempt to silence the speaker.

Academics worried about attacks on free speech have felt the need to respond, and they have articulated sound principles. Princeton professors Robert P. George and Cornel West recently attracted lots of supporters for a statement underscoring that "all of us should seek respectfully to engage with people who challenge our views" and that "we should oppose efforts to silence those with whom we disagree—especially on college and university campuses."

The issue, however, isn't whether the occasional conservative, libertarian or religious speaker gets a chance to speak. That is tolerance, an appeal to civility and fairness, but it doesn't take us far enough. To create deeper intellectual and political diversity, we need an affirmative-action program for the full range of conservative ideas and traditions, because on too many of our campuses they seldom get the sustained, scholarly attention that they deserve.

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